The present invention relates to the field of drilling boreholes. More particularly, the present invention relates to an improved drill system for generating slender boreholes in geologic formations, and particularly for the purpose of creating geophysical shot holes.
Geophysical seismic operations use explosive charges to generate source signals in the form of shock waves for penetrating subsurface geologic formations. In land-based geophysical seismic operations, slender shot holes several inches in diameter and ranging five to several hundred feet deep are drilled into near surface geologic formations. Explosive charges are positioned within the shot holes, and the explosive charges are detonated to generate the shock waves. The shock waves are reflected from subsurface geologic structures and interfaces, and the reflected energy is detected with receivers or geophones located at the surface. Transducers reduce the reflected energy into signals which are recorded for processing.
Conventional rotary or reciprocating drills can generate shot holes in unconsolidated soils such as topsoil and clay layers. However, conventional drilling equipment is particularly challenged by hard rock. Hard rock drills optimized for hard rock drilling are inefficient in unconsolidated soils because the drill mechanism is fouled by clay materials.
Numerous alternative systems have been developed to generate boreholes in rock. Such systems include water jet assisted drilling, thermal spalling systems, explosive capsule drills, liquid explosive drills, shaped and gauge charges, and combination rotatary and explosive systems. U.S. Pat. No. 3,670,828 to Bennett (1972) used shaped explosive charges detonated by a firing mechanism to impact the geologic formations. U.S. Pat. No. 3,601,061 to Dardick (1971) disclosed a light gas hypervelocity gun. A primary ammunition round having a propellant charge operated in combination with a piston and a secondary ammunition piece. Gas was introduced under pressure into a piston and the primary round was fired.
One tool known as the Tround drilling tool combined a projectile firing gun barrel with a conventional drill bit. The term "Tround" referred to triangular rounds of ammunition and was described in U.S. Pat. No. 3,855,931 to Dardick (1974) as using projectile rounds having a rear chemical propellant charge for accelerating the projectile against the geologic formations. U.S. Pat. No. 4,004,642 to Dardick (1977) described small caliber projectiles fired to generate shock wave interaction, and the utilization of residual gun gases to actuate drill heads and reamers. Mechanical pulverizing teeth pulverized the fractured rock, and this drilling technique increased drilling rates two to five times over conventional drilling systems. A variation of this system was disclosed in U.S. Pat. No. 4,582,147 to Dardick (1986), wherein projectiles were fired away from the tool center to fracture the geologic formations toward one side, thereby causing the drill bit to deflect toward the fractured rock. By controlling the location of the projectile impact relative to the drill head, the direction of the drilling could be controlled.
Another high speed electromagnetically accelerated earth drill was disclosed in U.S. Pat. No. 4,997,047 to Schroeder (1991). Metal ringed, frozen water projectiles were accelerated by a electromagnetic ringing circuit formed with multiple toroidal accelerating coils mounted transverse to the barrel axis. Each projectile included at least two metal rings around the frozen ice core. The multiple accelerating coils were powered with a direct current charger comprising storage batteries charged with a 110 volt battery charger. Alternatively, Schroeder stated that homopolar generators could charge the accelerator coils. The system required multiple timing circuits, projectile position indicators, and variable capacitance storage to effectively phase the coil activations. Alternating polarity between adjacent coils pushed and pulled the projectiles through the barrel.
A recently developed system for breaking rock was disclosed in U.S. Pat. No. 5,474,364 to Ruzzi (1995), wherein shotgun cartridges or other firearm ammunition provided the explosive charge for moving a steel rod to break the rock. In addition to this approach, railgun thrusters have been developed to move projectiles and to generate thrust. U.S. Pat. No. 5,439,191 to Nichols et al. (1995) disclosed a satellite thruster system for generating a high velocity plasma jet. A high energy pulse source was connected to a coaxial or dual-rail accelerator, and a heated propellant plasma was accelerated by a magnetic field. This magnetic field is generated by the rails which generate a "Lorenz" magnetic force perpendicular to the magnetic field on the plasma. The magnetic field accelerates a plasma through the parallel rails in one direction, and the acceleration magnitude depends on the rail length and the current provided.
The power systems for railguns typically comprise capacitor banks, and the wave form for such power is controlled by varying the inductor of the pulse forming network. Current flow through the railgun is induced through electrodes such as rails, creating the electromagnetic field in the railgun bore. Higher current with a shorter discharge duration results in a higher projectile velocity. Additionally, higher current increases the railgun efficiency because the ratio of projectile kinetic energy increase rate to the railgun energy consumed per unit time increases with higher current. The size of the power source and desired thrust performance are relevant to the successful application of railguns to a particular use.
Conventional systems for drilling a borehole through geologic formations such as hard rock have been limited by various factors. The narrow confines within a slender borehole significantly limits downhole placement of equipment and power systems. There is, accordingly, a need for an improved system capable of drilling a slender borehole through subsurface geologic formations. The system should be transportable into remote areas and should efficiently generate a borehole through unconsolidated soils and hard rock.